The invention relates to a pressure limiting valve for protecting hydraulic units, especially protecting the hydraulic face support of underground operations against rock bursts or other overloads, comprising a control piston that is movable inside an internal bore in the valve casing against the force of a valve spring, where said control piston blocks off the adjustment screw bore associated with the spring chamber and cooperates with a valve piston with blind bore and radial bores, which is guided in a movable manner in an expanded large bore that merges into a compensation chamber and is sealed via sealing rings, and which, when subjected to pressure, connects the cross-bores serving as discharge bores with the large bore by driving over the sealing rings.
Such pressure limiting valves, also called rock burst valves, are used where the occurrence of an overload would threaten to damage the hydraulic support in underground hard coal mining. The hydraulic props, both as individual props and in support frames, are equipped with such pressure limiting valves in order to prevent permanent damage or even destruction in the case of an overload, and thus to prevent such threats to the miners. German Patent 28 30 891 reveals a pressure limiting valve in which overpressures in the hydraulic system are reduced via a valve spring located between a plug screw and valve piston. A conical or spherical closure element that is lifted from the valve seat when an overload occurs is shaped to the valve plate or spring plate. A damping cylinder that restricts the passage opening is shaped to the piston. These known pressure limiting valves do not exhibit a sufficient closing safety. In addition, a correct design of the spring in particular is very difficult, but represents the precondition for a safe response for such a rock burst valve.
Further developments of this kind are found in German Patent 33 14 837, German Patent 39 22 984, and also German Patent 39 29 094. The two latter pressure limiting valves exhibit a construction that permits a rate of flow of approximately 1,000 l/min, whereby the valves are markedly small. This is accomplished by constructing the valve piston as a tubular sheath that cooperates with a control piston, so that both can be moved together as a structural unit inside the internal bore of the valve casing. The known control piston exhibits a blind bore and radial bores through which the pressure medium is able to flow into the spring chamber when the valve responds. The necessary differential surfaces are thus realized with the stepped control piston or the different diameters of control piston and valve piston. The disadvantage in this pressure limiting valve and in the one known from German Patent 39 29 094 consists of the still insufficient closing values, whereby in particular a reclosing of the valve and thus a fluttering of the valve piston occurs, since both pistons must be moved simultaneously. The valve spring being used must be accordingly large and long, requiring a more complex construction for such pressure limiting valves.